Reference beam grade control for asphalt pavers

ABSTRACT

The disclosure relates to an improved type of grade control for an asphalt paver utilizing reference members positioned in leading and trailing relation to the pavement-forming screed. Control over the grade of the asphalt mat laid down by a floating screed is controlled by the combined action of the reference members moving along with the paving apparatus and sensing the average grade conditions of the base in front of the screed as well as the average grade conditions of the newly laid pavement mat. The new control provides for exceptional smoothness and uniformity of the pavement mat, and also greatly facilitates the use of mobile grade reference controls when laying extremely wide pavement mats.

United States Patent [191 Davin 1 51 Nov. 5, 1974 [75] Inventor: DonaldR. Davin, Shelbyville, 111.

[73] Assignee: Blaw-Knox Construction Equipment, Inc., Mattoon, 111.

22 Filed: Mar. 26, 1973 21 Appl. No.2 345,115

[52] [1.8. Cl. 404/84 [51] lint. Cl. E0lc 19/00 [58] Field of Search404/83, 84; 37/DlG. 2O

[56] References Cited UNITED STATES PATENTS 2,491,275 12/1949 Millikin404/84 2,962,979 12/1960 McCormick 404/84 X 3,259,034 7/1966 Davin404/83 3,272,099 9/1966 Drake 404/84 3,323,427 6/1967 Schrimper3,595,144 7/1971 Rink 404/84 1 /0 1, mm /4 26 'T 3,618,484 11/1971 Long404/84 Primary ExaminerNile C. Byers, Jr.

Attorney, Agent, or Firm-Mandeville and Schweitzer [57] ABSTRACT Thedisclosure relates to an improved type of grade control for an asphaltpaver utilizing reference members positioned in leading and trailingrelation to the pavement-forming screed. Control over the grade of theasphalt mat laid down by a floating screed is controlled by the combinedaction of the reference members moving along with the paving apparatusand sensing the average grade conditions of the base in front of thescreed as well as the average grade conditions of the newly laidpavement mat. The new control provides for exceptional smoothness anduniformity of the pavement mat, and also greatly facilitates the use ofmobile grade reference controls when laying extremely wide pavementmats.

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\ mik REFERENCE BEAM GRADE CONTRQL FOR ASPHALT PAVERS BACKGROUND ANDSUMMARY OF INVENTION In the laying of asphalt pavement roadways and thelike, it is widespread practice to employ so-called floating screedpaving machines. These machines include a tractor-like main frame havingan engine for propulsion and for material distributing functions.Typically, there is a material receiving hopper at the front of thepaver arranged to receive hot asphalt material from a truck, as thepaving machine advances along the roadbed. Slat conveyors or thelike areprovided to convey the material from the hopper, at the front of themachine, toward the floating screed, at the back of the machine.Immediately in front of the screed, there is typically provided adistributing auger, which receives the raw asphalt material from theslat conveyor and conveys it laterally so as to distribute the materialalong the front edge of the screed. As the machine advances along theprepared roadbed, the raw asphalt material flows under the screed, whichlevels, smooths and compacts it to provide a continuous, level pavementmat.

In a typical floating screed asphalt paver, the screed is attached to apair of forwardly extending tow arms which engage the paver frame attheir forward extremities. These tow arms are also connected to thepaver frame by hydraulic or other actuators arranged to adjust thevertical position of the tow arm extremities in relation to the paverframe. By effecting proper control over the position of the tow armforward extremities in relation to a reference plane or a referenceelement, substantially independent of the irregular vertical motions ofthe paver frame itself, it is possible to cause the floating screed tolay a pavement mat which is smooth and level in relation to theunderlying base surface.

Effective control of the screed tow point adjustment may be achieved bymeans of suitable feeler device, for example, which is carried by one orboth of the tow arms and arranged for contact with a predeterminedreference surface. When the tow point becomes either higher or lowerthan is indicated by the reference surface, as with changing loads uponpaver frame and/or irregularities in the roadbed surface, the tow pointis caused to be controllably raised or lowered relative to the paverframe to maintain a constant relationship with the reference. Such anarrangement is shown in the Ackerman, et al., U.S. Pat. No. 3,236,163,for example. ln many applications, grade control of the tow point isprovided at only one side of the machine. For controlling the tow pointat the other side there typically may be provided a so-called slopecontrol, which functions to maintain a constant relationship between towpoints at opposite sides, either on a level basis or with apredetermined transverse slope.

in conjunction with the type of tow point control mentioned in thepreceding paragraph, it is important to provide an appropriatereferencefor the feeler device. In some cases, the reference maybe theedge of previously laid pavement mat, in which case a simple shoe plateor the like is arranged to be towed along the pavement surface. in othercases, it may be necessary or advantageous to position a reference wirealongside the base surface, in a position to be engaged by the sensingdevice, such that the screed tow arms are maintained in a constantrelationship to the reference wire.

In some instances, it is undesirable or inconvenient to utilize areference wire or a previously laid pavement surface as a gradereference. For such occasions, it has proven advantageous to utilize amobile reference beam, which is carried along with the paver as it movesover the roadway base surface. An arrangement of this type is describedand claimed in the D. R. Davin U.S. Pat. No. 3,259,034. In thearrangement of the Davin patent, an elongated beam structure is providedwith a substantial plurality (e.g., ten) of independent supports,advantageously arranged on individual springs. The arrangement is suchthat, as the reference beam is carried along the base surface, it issupported by the combined action of the multiple, yieldable supports.The individual supports are enabled to follow the minor deviations inbase contour without significantly affecting the position of thereference beam as'a whole, and the mobile reference beam thus provides asuitably accurate, averaged reference plane representing the grade towhich the pavement mat is to be applied. A sensing device carried by thescreed tow arms engages the reference beam near its center, to enablethe tow points to be maintained in a predetermined relationship to themoving reference beam.

While the arrangement illustrated in the D. R. Davin U.S. Pat. No.3,259,034 constituted an important advance, certain problems wereencountered in using the specific apparatus of that patent withconventional paving machinery when laying extremely wide mats of pavingmaterial, as is becoming increasingly prevalent with the availability oflarger and more powerful paving machines. In part, these problems derivefrom the fact that, in order to be suitably accurate, a moving referencebeam must have substantial length, typically greater than that of thepaving machine itself. When paving narrow widths, this presents noproblem, as the beam may extend slightly ahead of and slightly in backof the paving machine, while being towed alongside of it by a suitabletow bracket extending laterally from the frame of the paver. However,where the floating screed and the associated material distributingaugers are substantially wider than the paver frame, it becomesextremely difficult, if not impossible, to effectively tow the referencebeam and to derive an accurate reference signal from a locationoutwardly of the lateral extremities of the screed and auger. Merelymoving the reference beam forward, so as to be entirely in front of theauger and screed is not entirely satisfactory, moreover, because ofpractical difficulties in deriving a grade level signal reflecting theposition of the tow arms in relation to the geometrical center of thereference beam.

in accordance with the invention, the utilization of a moving referencebeam arrangement in connection with the laying of wide pavement mats ismade feasible by the utilization of a unique combination of referencebeams, one being towed ahead of the screed and auger, supported on theroadway base grade, and the other being towed behind the screed andauger, supported on the just-laid asphalt mat. To greatest advantage,the arrangement of the invention includes a system of compound leversassociated with the respective leading and trailing reference beams andarranged to derive a signal which is a function of the relationship ofthe screed and its tow arms to the respective reference beams. lneffect, the reference beam arrangement is of greatly increased lengthand enables the laying of a mat of increased smoothness and accuracy incomparison to prior arrangements.

To advantage, the trailing reference beam, which is towed by the paverframe behind the auger and screed is supported by one or more wheels orin some cases by an elongated flexible slide strip. Moreover,notwithstanding that the trailing reference beam rides upon the justdaidmat, it is desirable in many cases that the trailing reference beam besupported by a large plurality of wheels arranged for independentmovement, such that the trailing reference reflects an averagedreference position. In the case of either the leading or trailingreference beams, or both, it is considered preferable to utilize a rigidbeam with independently yieldably movable supports. However, it iswithin the purview of the invention to utilize reference beam structurescomprising articulated pairs of supports (e.g., see DeLeuze, French Pat.No. 1,056,865).

In its most advantageous form, the system of the invention includesleading and trailing reference beams of rigid structure, independentlysupported by a plurality of yieldable supports. The leading referencebeam is supported by a plurality of shoes or plates, while the trailingbeam is supported by a plurality of wheels. Elongated reference armsextend rearward from the leading beam and forward from the trailing beamand are pivotally connected to a screed tow arm, advantageously at apoint forward of the screed itself but well behind the tow point. Thus,the reference arm pivot point is in a position to reflect deviationsfrom the reference level of both the tow point and the screed itself.

For a better understanding of the invention andits various features andadvantages, reference should be made to the following detaileddescription and to the accompanying drawings, with further reference tothe appended claims.

DESCRIPTION OF THE DRAWING FIG. 3 is an enlarged elevational view,similar to FIG. I

2, illustrating a preferred form of yieldable support for a trailingreference beam.

FIG. 4 is a top plan view illustrating features of the trailingreference beam utilized in the apparatus of FIG. I, and showingreference beams utilized on both sides of the paving machine.

FIG. 5 is an enlarged, fragmentary elevational view illustrating detailsof the tow point suspension and reference signal means incorporated inthe paving apparatus of FIG. 1.

DESCRIPTION OF A PREFERRED EMBODIMENT Referring now to the drawings, andinitially to FIG. 1 thereof, the reference numeral designates generallya paver frame of a floating screed type asphalt paver. The paver per seis well known in principle and includes an engine or prime mover 11which provides a source of tractive power and also provides power forthe various material conveying and distributing functions of themachine. In the illustrated apparatus, a pair of large, pneumatic tires12 at the back of the paver provide the necessary forward traction, withsteerage and support for the front of the paver being provided by pairsof smaller wheels 13.

In front of the paver 10, there is conventionally provided a hopper 14arranged to receive paving material, which may be aggregate, asphalt,and the like, from the tilted body of a truck (not shown). In accordancewith conventional practice, the truck is brought into contact with thefront of the paver, and then is pushed along by the paver, continuouslydischarging its contents into the hopper 14 during the progress of thepaving operation, until the complete truckload is exhausted. Thereafter,the empty truck is replaced by a new, fully loaded truck, with thepaving operation continuing from the hopper supply during the changeoverinterval.

By means of a suitable slat conveyor (not shown) the paving material isconveyed from the hopper 14 to the back of the machine and deposited infront of a controllably rotated auger 15. The auger is pitchedoppositely from the center, so as to convey the conveyordischargedpaving material laterally outward and distribute it more or less evenlyalong the full length of the auger. In this respect, it will beunderstood that the basic body of the paver frame 10 may have an overallwidth of 10 or 12 feet, for example, to accommodate its over-the-roadtransportation from place to place. At the same time, the paver may beand often is set up in a configuration to lay paving mats in anuninterrupted width of 20 to as much as 40 feet on occasion. In suchcases, the overall width of the auger 15 is substantially equal to thefull paving width.

Disposed immediately behind the auger 15 is a strikeoff and screedstructure generally designated by the numeral '16, which is carried by apair of forwardly extending tow arms 17. The screed assembly 16, likethe auger 15, has a width corresponding to the desired paving width, andthus may be substantially wider than the width of the paver frame 10.The tow arms 17, are spaced so as to be closely adjacent to sides of thepaver frame. Accordingly, the tow arm 17 may engage the screed assembly16 well inboard of its lateral extremities. Intermediate portions 18 ofthe tow arms extend upward and over the top of the area occupied by theauger 15.

In accordance with conventional practice, the tow arms 17 may beconnected to the paver frame by means of spaced tow pins 19, 20 (FIG. 5)which are freely slidable on a vertically disposed tow bar 21. Anhydraulic actuator 22 is mounted at the upper end of the tow bar 21, andextends downward with its rod 23 engaging the forward portion of the towarm 17. By appropriate control of the actuator 22, the forward extremityof the tow arm may be appropriately raised or lowered in relation to thepaver frame itself to effect desired adjustments in the angular attitudeof the tow arms 17 and the screed assembly 16. Similar arrangements are,of course, provided at both sides of the machine, with the respectiveactuators being separately controllable, however, to provide forindependent manipulation of tow arm elevation on opposite sides of themachine.

In accordance with well known principles, a smooth, level mat 24 ofpaving material may be laid by the screed assembly 16 more or lessindependently of variations in the base roadbed 25 and also more or lessindependently of changes in the suspension of the paver frame itselfresulting from changing loads in the hopper 14, for example, or movementof the wheels into or over minor discontinuities or obstructions in theroadway. This is realized in part by providing a socalled gradereference level, which is independent of the paver frame 10, and bymaintaining the tow point of at least one of the arms 17, at apredetermined height in relation to that reference. The other tow armlikewise may be controlled by a similar reference means, although it isusually more common to control one of the arms from a grade referenceextending longitudinally of the roadway while controlling the other towarm by means of a so-called slope control. If the slope angle is zero,both tow arms will automatically be adjusted to maintain an equaluniform height in relation to a single grade reference. Frequently,however, a predetermined cross slope is built into the pavement surface,to facilitate drainage and/or for banking at turns. In the latter case,both tow arms can be adjusted in accordance with a single gradereference means, but the two tow arms will be maintained in an unequallyspaced relation to that grade reference to provide the desired crossslope.

In accordance with the teachings of the beforementioned Davin US. Pat.No. 3,259,034, a suitable artificial grade reference frequently may bederived from the base roadbed itself, by means of an elongated referencebeam individually supported by a large plurality of independentlyyieldable supporting elements. The base road mat 25 may be a preparedbut unpaved base, or

may even be a previously laid asphalt course, where thefinished pavementmat consists of more than one asphalt course. The present inventionutilizes to a large extent some of the important principles of thebeforementioned Davin patent, while at the same time providing a greatlyimproved mobile reference beam arrangement which is suitable forpavement configurations in which the screed and auger are ofsubstantially greater overall width than the paver frame itself-andwhich, at the same time, provide for an exceptionally high degree ofaccuracy in the control of the level and smoothness of the resultantpavement mat.

In accordance with the invention, the paving machine is provided withdual mobile reference beams 26,

27, carried alongside and closely adjacent to the paver frame 10, withone reference beam 26 being carried in leading relationship to theauger-screed area and the second reference beam 27 being carried intrailing relationship. At least the forward beam 26, and to bestadvantage each of the mobile reference beams, is of highly extendedlength, possibly greater than the length of the paver itself. inaddition, at least the leading beam 26, and advantageously both beamsare supported at a multiplicity of points along its length by equallyspaced, independently movable supporting elements. The arrangement issuch that the reference beam is supported in a positionwhich representsan average condition of the underlying surface over an extended length.The function and operation of the forward or leading mobile referencebeam 26 is in many ways similar to the mat, immediately behind thescreed assembly 16. By an advantageous arrangement of reference arms orlevers, reference data information from the respective beams 26, 27 iscombined and made available to a sensing device 28 to effect controlledmovement of the actuator 22 and thereby maintain a constant relationshipbetween the screed assembly 16 and the respective leading and trailingreference beams 26, 27.

In the illustrated form of the apparatus, the moving reference beam 26is in the form of a three-piece lightweight tubular beam joined atflanges 29 into a unitary assembly. The segmented construction permitsdisman tling for transportation, as will be understood. In its assembledform, the leading reference beam 26 may have a length on the order of 30feet and is supported at uniformly spaced intervals by a large plurality(10 in the illustration) of yieldably mounted shoes 30 (FIG. 2). Toadvantage, the shoes 30, may be made of flat sheet metal, pivotallyconnected at 31 to a yoke bracket 32 carried at the end of a guide bolt33. A stabilizing link 34 is pivotally connected to the yoke 32 andextends forward to a lug bracket 35, to which it is pivotally connectedat 36. The bolt 33 is slidably guided in a spherical bearing 37 receivedin a boss 38 formed on the reference beam 26. A spring 38 extendsbetween the spherical bearing 37 and the top of the yoke 32, and isarranged to be compressed as a function of upward movement of the shoe30 relative to the reference beam 26.

The arrangement of the several supporting shoe assemblies for theleading reference beam is such that, when the beam is resting on thebase surface 25, in its normal operating arrangement, the springs 38 ofall of the shoe assemblies are partially compressed. And, although thebase surface 25 may not be precisely level throughout the length of thereference beam, and may contain minor aberrations in the form of smallrises or depressions, the beam itself and particularly its midpoint,will tend to maintain an averaged position above the roadway, reflectingthe average condition of the surface. As will be understood, individualshoe assemblies may rise or fall relative to the beam in followinganomalies in the road surface. The reference beam itself will beinfluenced only to a minor extent by individual movements of the shoeassemblies, but will reflect an average condition. The geometric centerof the beam, in terms of the location of the supports therefor, reflectsmost precisely the average reference condition sought to be determined.

In the system illustrated in the Davin US. Pat. No. 3,259,034 patent,the geometric center of the beam is located near the forward extremityof one of the tow arms, and a sensing element is provided to detect therelationship between the forward portion of the tow arm and thegeometric center area of the beam. In the system of the invention,however, since the leading reference beam 26 is to be located entirelyforward of the auger 15, the geometric center region of an adequatelylong reference beam is located far ahead of even the forwardmostextremity of the tow arm 17. Thus, in accordance with one aspect of theinvention, the leading reference beam 26 is provided in its effectivecenter region with a vertically extending bracket 39 which is pivotallyconnected at 40 to an elongated, rearwardly extending reference arm 41.In the illustrated system, the

- reference arm 41 includes an upward extension 42 which is pivotallyconnected at 43 to the screed arm 17.

Desirably, the pivot point 43 connecting the reference arm 41 to screedtow arm is located rearward of the tow bracket 21 and at the same timewell forward of the screed assembly 16. A point approximately midwaybetween these areas is advantageous, inasmuch as vertical movementrelative to the desired reference of either the screed assembly I6 orits tow point will be reflected in vertical movement of the pivot point43 and a corresponding vertical movement of the reference arm 41.

The reference beam 26 is carried along with the paver frame by means ofa forward tow linkage 44, by which the beam is attached to the front ofthe paver. An articulated stabilizing linkage 45 connects the back ofthe reference beam 26 to a portion 46 of the paver frame (FIG. 5) toprovide lateral stability.

The trailing reference beam 27 typically may be somewhat shorter inlength than the leading beam 26. For example, a beam length of feet forthe trailing beam may be suitable, while a 30 foot length on the leadingbeam would be preferable. In the illustrated arrangement, the trailingbeam is constructed in two sections, bolted together at a center flange47. To greatest advantage, the trailing reference beam 27 is supportedby a large plurality of uniformly spaced sets of elements, there beingeight such sets in the specific illustration. Under some conditions, asingle support element might suffice for the rear reference, but aplurality is required and/or desired in most instances. Wheels are usedrather than flat shoe plates, as in the case of the leading referencebeam, because the newly laid asphalt mat 24 is still in a soft conditionwhen traversed. by the reference beam andcould be scraped or marred byindividual sliding shoes. Most advantageously, the wheels 48 arearranged in pairs for increased flotation on the soft mat surface, andalso to impart stability to the beam to facilitate its handling whendetached from the paver.

Notwithstanding that the newly laid pavement mat 24 may have a highdegree of smoothness and levelness, it is advantageous in most casesthat the plurality of wheel pairs 48 for the trailing reference beam beindependently supported, so that the position of the beam reflects anaverage position of the multiplicity of wheel pairs. To this end, thewheel pairs may be supported much in the same manner as the shoes 36 ofleading reference beam. Thus, an axle 49 connecting the two wheels of apair is journalled in a yoke bracket 50 carried at the end of a guidebolt 51 slidably received in a spherical bearing 52. A coil spring 53urges the yoke bracket 50 downward. A stabilizing linkage 54 extendsforward from the bracket and is pivotally connected to a fixed lug 55extending downward from the bottom of the beam 27. The arrangement issuch as to freely accommodate individual, yieldably resisted verticalmovement of the wheel pairs relative to the rigid beam.

In some cases, it may be appropriate to provide the trailing beam withnon-rotating supports, like those of slide strip is suitably secured atits front end to the reference beam or to the paver itself, so as to betowed along with the beam. The strip readily flexes to accommodate thedesired independent movement of the beam supports.

The trailing beam advantageously is towed from the screed assembly 16,by means of a pivoted tow link 56 (FIG. 4) extending rearward from abracket 57 suitably attached to the screed. For lateral stabilization ofthe back of the trailing beam 27, there is provided a diagonalstabilizing linkage consisting of a tie rod 58 secured to the beam by aclamp 59 and extending forwardly to a connection point at the oppositeside of the machine. In the arrangement shown in FIG. 4 of the drawings,the equipment is set up to employ reference beam systems on both sidesof the paving machine. In that case, the stabilizing bar 58 for thereference beam 27 may be secured by a clamp 60 to the forward portion ofthe opposite side trailing reference beam 270. Likewise, the beam 27awill be stabilized by a tie bar 58a secured by clamps 59a and 60a. Whereonly a single trailing reference beam is utilized, the stabilizing bar58 can be connected more directly to the screed assembly 16.

In the effective center region of the beam 27, in terms of the locationof the uniformly spaced supports, there is provided a verticallydisposed extension bracket 61 which pivotally engages at 62 the trailingend of a reference armstructure 63. The reference arm structure 63desirably is constructed in the form of a truss arranged to support anelongated, forwardly extending reference arm element 64. The specificconfiguration of the truss 63 is unimportant, apart from the fact thatit must be consistent with its passing over the top of the screed andauger structures at the back of the paver.

In the illustrated arrangement, the reference arm 64 is attached by thepivot pin 43 to the forward reference arm 41 and to the screed tow arm17. Within the purview of this invention, the forward and rearwardreference arms 41, 64 could be separately pivoted to the tow arms 17, orthey could be pivotally connected together and pivoted to the tow arm 17at a different axis. However, a common pivot point at the pin 43 issimple and advantageous.

As is reflected in FIGS. 1 and 5, the rearward reference arm 64 extendsforwardly well beyond the pivot pin 43, to a region locatedapproximately over the center of the forward reference arm 41. Thesensing device 28, typically in the form of a potentiometer actuated bya feeler element 65, is secured to the upper reference arm 64, by meansof a clamping bracket 66. The bracket 66 may be secured in any ofseveral positions along the length of reference arm 64, so as to beproperly positioned with respect to the forward reference arm 41. Inaddition, the sensing device 28 has a vertically extendable support 67adjustably secured in the clamp 66. As reflected in FIG. 5, the sensingdevice 28 may be vertically adjusted in the clamp 66 to a position inwhich the feeler element bears upon the center region of the forwardreference arm 41. When properly adjusted, the feeler element 65 will beapproximately midway between its upper and lower extreme positions whenthe proper space relationship exists between the respective referencearms 41, 64. Any change in the spacing between these arms, in the regionof the sensor 28, will cause a displacement of the feeler 65, eitherupward or downward. In accordance with principles which are well knownand need not be repeated here, movement of the potentiometer feelerelement 65 away from its neutral position can be utilized to effectenergization of the actuator 22 in a direction that will tend to restorethe feeler to its neutral position by appropriate upward or downwardmovement of the tow arm 17, as may be necessary.

In typical operation, the reference arms 41, 64 are arranged to be moreor less horizontal and parallel when the respective leading and trailingreference beams are in a desired, predetermined relationship and the towarms 17 are properly positioned. To this end, the vertical supports 39,61 may be provided with a plurality of openings or other adjustmentfacilities for establishing a desired level for the pivot points 40, 62of the respective reference arms.

When the system is in operation, the paver frame advances forwardly (tothe left in FIG. 1) carrying with it the reference beams 26, 27, theformer riding on the roadway base surface and the latter riding on thejust laid pavement mat. Assuming, for example, that the level of thepaver frame 10 were to drop, relative to the reference, as by reason ofan increased load in the hopper and/or one or more of the wheels movinginto a minor depression in the roadway, the forward extremity of the towarm 17, being attached to the paver frame through the hydraulic actuator22, would be correspondingly lowered relative to the reference beams 26,27. This would in turn cause a lowering of the pivot point 43 and aresulting relative closing of the distance between the reference arms41, 64 in the region of the sensor 28. Immediately, the actuator 22would be energized to raise the forward end of the tow arm 17 suffivciently to re-establish proper spacing between the reference arms. As aresult, the orientation of the tow arms 17 to the roadway base surface25 and to the pavement surface 24 is retained substantially constant,notwithstanding vertical deviations of the paver frame itself.

As will be appreciated, the screed assembly 16 is supported by flotationon the viscous asphalt paving material as the paver is advanced on theroadway. The viscosity and other characteristic nature of this materialideally should be constant at all times. As a practical matter, however,the consistency of the mixture may vary from truckload to truck load,and the viscosity characteristic of even the same mixture may varysomewhat as a function of temperature, for example. Accordingly,assuming the paving frame 10 itself is traveling a perfectly levelcourse, there may be some tendency for the screed assembly 16 to deviateupward or downward in some measure from the desired paving level. Whenthis occurs, it is reflected in upward or downward movement of the pivotpoint 43 and a corresponding change in the relationship of the referencearms 41, 64. For example, if the screed assembly would tend to sink intothe mat, as a result of a lowering of the material viscosity, areduction in forward speed, or like circumstance, the pivot point 43would be correspondingly lowered, and the distance between the referencearms and the region of the sensor 28 would be correspondingly reduced.The potentiometer feeler 65 would be displaced from its neutralposition, energizing the actuator 22 and raising the forward extremityof the tow bar 17 to re-establish the proper spacing. In this respect,raising of the forward end of the tow bar would increase the angle ofincidence of the screed bottom plate 68 with respect to the pavementmat, tending to cause the screed assembly 16 to seek a higher level andthereby re-establish the desired level of the pavement surface.

The dual reference beam system of the invention results in anexceptionally high degree of accuracy and smoothness in the laying of anasphalt paving mat, because the level of the screed .plate is controlledas a combined function of the roadway base surface 25 and the just-laidpavement surface 24. Moreover, the extreme overall length of the dualreference beam system provides for exceptional overall responsiveness ofthe control to deviations from a desired level surface.

In addition to the foregoing advantages, the system of the inventionprovides a rather simplified and wholly practical arrangement forutilizing a mobile reference means in conjunction with heavy dutyasphalt pavers having the capacity to lay a pavement mat of far greaterwidth than that of the paver frame itself. This is of particularimportance in view of current trends toward the paving of 30 and 40 footmat widths in a single pass with the paving machine.

In a most advantageous form of the invention it is provided that thetrailing reference beam, although riding on an essentially smooth andlevel, just-laid pavement surface, is supported by a large plurality ofelements, preferably wheels, each being independently supported. in thisrespect, notwithstanding the essentially smooth, flat characteristic ofthe newly laid pavement surface, wheels can, in continued operation,become rather irregular through non-uniform pickup of adherent asphaltmaterial. By reason of the multiple, independent support of theplurality of wheels, the trailing reference beam can be supported in anappropriately averaged position notwithstanding a degree ofout-of-roundness of the individual wheels during normal operations.

It should be understood, of course, that the specific form of theinvention herein illustrated and described is intended to berepresentative only, as certain changes may be made therein withoutdeparting from the clear teachings of the disclosure. Accordingly,reference should be made to the appended claims in determining the fullscope of the invention.

I claim:

1. A dual reference beam grade control system for i use in combinationwith an asphalt paver of the type having a transversely disposedmaterial distributing auger, a floating screed positioned behind theauger, tow

' arms extending forwardly from the screed and connected to the paverframe, and tow arm suspension means for vertically adjusting the towpoint connections of said tow arms to effect changes in the attitudeangle of the screed, said grade control system comprismg a. a beam-likefirst reference member towed by the paver frame and extendinglongitudinally along one side thereof,

b. said first reference member being disposed entirely forward of theauger,

c. a plurality of independently movable supports for said firstreference member spaced along its length and supporting said referencemember at an averaged position above a base grade surface,

d. a first reference arm pivotally connected to the first referencemember in its central region and extending rearwardly toward and beingpivotally connected to one tow arm,

e. a second reference member towed by the paver frame and being disposedentirely behind the screed,

f. support means for supporting said second reference member above anasphalt mat grade surface,

g. a second reference arm connected to the second reference member andextending forwardly toward and being pivotally connected to said one towarm and/or said first reference arm, and

h. tow arm height reference control means associated with each of saidreference arms and operatively associated with said tow arm suspensionmeans for raising and lowering the tow point of said one tow armrelative to the paver frame in response to relative movements of one orboth of said reference arms in relation to said one tow arm.

2. The grade control system of claim 1, further characterized by a. saidreference control means including means engaging both of said referencearms and operative to detect relative movement therebetween.

3. The grade control system of claim 2, further characterized by a. saidcontrol means including a sensing potentiometer or the like carried byone of said reference arms and engageable with the other to detect relative movement.

4. The grade control system of claim 1, further characterized by a. saidreference arms being pivotally connected to said one tow arm,

b. one of said reference arms having a portion extending longitudinallyfor a substantial distance beyond its pivot axis in the direction of theother reference arm, and

c. said reference control means being operatively associated between theextending portion of said one reference arm and a portion of said otherreference arm.

5. The grade control system of claim 4, further characterized by a. saidsecond reference arm having a portion extending longitudinally forwardof said axis.

6. The grade control system of claim 5, further characterized by a. saidreference arms being connected to said one tow arm along a common pivotaxis.

7. The grade control system of claim 6, further characterized by a. saidpivot axis being located on said one tow arm between the tow point andthe screed and closer to the screed than the tow point.

8. The grade control system of claim 1, further char- 10. The gradecontrol system of claim 9, further characterized by a. the supportelements for said second reference member comprising wheels. 11. Thegrade control system of claim 10, further characterized by a. each ofsaid support elements comprising a pair of axially spaced wheels,positioned on opposite sides of the reference member. 12. The gradecontrol system of claim 10, further characterized by a. the supportelements for the first reference beam comprising bearing plates slidablyengageable with the base reference. 13. The grade control system ofclaim 9, further characterized by a. flexible slide strip meansinterposed between the asphalt mat surface and each of the supportelements for the second reference member.

14. The grade control system of claim 1, further characterized by a.said first reference member being towed from the paver frame from apoint substantially forward of the back end of the member,

b. stabilizing linkage means being connected between the paver frame andthe back portion of the first reference member,

c. the second reference member being towed from the paver frame from apoint in the forward region of the reference member, and

d. stabilizing linkage means being connected to the back portion of thesecond reference member and extending diagonally forwardly and towardthe opposite side of the back portion of the paver frame.

15. The grade control system of claim 14, further characterized by a.the combination comprising dual reference member grade control systemsat each side of the paver frame for separately controlling the tow pointpositions of the respective tow arms, and

b. the diagonally disposed stabilizing linkage means extending from theback portion of one of the second reference members to the forwardportion of the other thereof.

16. A reference beam grade control system for use in combination with anasphalt paver of the type having a transversely disposed floating screedtowed behind a paver frame and screed control means for maintaining apredetermined relationship of the screed to a reference, which comprisesa. a plurality of about six or more supporting wheels,

b. an elongated reference control member supported by said plurality ofwheels in an averaged position in relation to a surface,

c. resilient means mounting said wheels in a longitudinally alignedrelationship along said reference control member for independentvertical movement in relation to said reference control member,

d. means for towing said reference control member directly behind aportion of said screed, whereby all of the said wheels engage thefinished asphalt mat laid by said screed, and

e. means for actuating the screed control means in response topredetermined relative vertical moveb. said screed control means beingactuated by movements of said reference arm.

1. A dual reference beam grade control system for use in combinationwith an asphalt paver of the type having a transversely disposedmaterial distributing auger, a floating screed positioned behind theauger, tow arms extending forwardly from the screed and connected to thepaver frame, and tow arm suspension means for vertically adjusting thetow point connections of said tow arms to effect changes in the attitudeangle of the screed, said grade control system comprising a. a beam-likefirst reference member towed by the paver frame and extendinglongitudinally along one side thereof, b. said first reference memberbeing disposed entirely forward of the auger, c. a plurality ofindependently movable supports for said first reference member spacedalong its length and supporting said reference member at an averagedposition above a base grade surface, d. a first reference arm pivotallyconnected to the first reference member in its central region andextending rearwardly toward and being pivotally connected to one towarm, e. a second reference member towed by the paver frame and beingdisposed entirely behind the screed, f. support means for supportingsaid second reference member above an asphalt mat grade surface, g. asecond reference arm connected to the second reference member andextending forwardly toward and being pivotally connected to said one towarm and/or said first reference arm, and h. tow arm height referencecontrol means associated with each of said reference arms andoperatively associated with said tow arm suspension means for raisingand lowering the tow point of said one tow arm relative to the paverframe in response to relative movements of one or both of said referencearms in relation to said one tow arm.
 2. The grade control system ofclaim 1, further characterized by a. said reference control meansincluding means engaging both of said reference arms and operative todetect relative movement therebetween.
 3. The grade control system ofclaim 2, further characterized by a. said control means including asensing potentiometer or the like carried by one of said reference armsand engageable with the other to detect relative movement.
 4. The gradecontrol system of claim 1, further characterized by a. said referencearms being pivotally connected to said one tow arm, b. one of saidreference arms having a portion extending longitudinally for asubstantial distance beyond its pivot axis in the direction of the otherreference arm, and c. said reference control means being operativelyassociated between the extending portion of said one reference arm and aportion of said other reference arm.
 5. The grade control system ofclaim 4, further characterized by a. said second reference arm having aportion extending longitudinally forward of said axis.
 6. The gradecontrol system of claim 5, further characterized by a. said referencearms being connected to said one tow arm along a common pivot axis. 7.The grade control system of claim 6, further characterized by a. saidpivot axis being located on said one tow arm between the tow point andthe screed and closer to the screed than the tow point.
 8. The gradecontrol system of claim 1, further characterized by a. said secondreference arm having a portion extending forwardly from its pivot axis,at least to a point generally vertically spaced from the first referencearm in a center region thereof midway between its respective pivot axesat opposite ends, and b. said reference control being operativelyassociated with said center region of the first reference arm and aportion of said second reference arm spaced vertically therefrom.
 9. Thegrade control system of claim 1, further characterized by a. said firstand second reference members comprising substantially rigid beam-likestructural elements of substantial length in relation to the length ofthe paver, and b. each of said beam-like structural elements beingsupported by a plurality of spaced independently movable supports, c.the several support elements for each structural element being operativecollectively to support the structural element in an averaged positionover its length, in relation to the reference surface engaged by thesupport elements.
 10. The grade control system of claim 9, furthercharacterized by a. the support elements for said second referencemember comprising wheels.
 11. The grade control system of claim 10,further characterized by a. each of said support elements comprising apair of axially spaced wheels, positioned on opposite sides of thereference member.
 12. The grade control system of claim 10, furthercharacterized by a. the support elements for the first reference beamcomprising bearing plates slidably engageable with the base reference.13. The grade control system of claim 9, further characterized by a.flexible slide strip means interposed between the asphalt mat surfaceand each of the support elements for the second reference member. 14.The grade control system of claim 1, further characterized by a. saidfirst reference member being towed from the paver frame from a pointsubstantially forward of the back end of the member, b. stabilizinglinkage means being connected between the paver frame and the backportion of the first reference member, c. the second reference memberbeing towed from the paver frame from a point in the forward region ofthe reference member, and d. stabilizing linkage means being connectedto the back portion of the second reference member and extendingdiagonally forwardly and toward the opposite side of the back portion ofthe paver frame.
 15. The grade control system of claim 14, furthercharacterized by a. the combination comprising dual reference membergrade control systems at each side of the paver frame for separatelycontrolling the tow point positions of the respective tow arms, and b.the diagonally disposed stabilizing linkage means extending from theback portion of one of the second reference members to the forwardportion of the other thereof.
 16. A reference beam grade control systemfor use in combination with an asphalt paver of the type having atransversely disposed floating screed towed behind a paver frame andscreed control means for maintaining a predetermined relationship of thescreed to a reference, which comprises a. a plurality of about six ormore supporting wheels, b. an elongated reference control membersupported by said plurality of wheels in an averaged position inrelation to a surface, c. resilient means mounting said wheels in alongitudinally aligned relationship along said reference control memberfor independent vertical movement in relation to said reference controlmember, d. means for towing said reference control member directlybehind a portion of said screed, whereby all of the said wheels engagethe finished asphalt mat laid by said screed, and e. means for actuatingthe screed control means in response to predetermined relative verticalmovements between the paver frame and said reference control member. 17.A reference beam grade control system according to claim 16, furthercharacterized by a. a reference arm being pivotally connected at one endto the mid-portion of said reference control member and connected atanother portion for pivotal movement in relation to said screed and forvertical movement as a function of vertical screed movement withreference to the mat surface, and b. said screed control means beingactuated by movements of said reference arm.